Investigation on Functionalized Ruthenium-Based Sensitizers to Enhance Performance and Robustness of Dye-Sensitized Solar Cells

It has been increasingly aware to the world today that reserves of fossil fuels are limited and their use has serious environmental side effects. Encouraged by this realization was the evolution of the use of cleaner alternative energy, among which Dye-sensitized solar cells (DSCs) are potentially attractive candidates for the lower cost of producing devices which convert an abundant amount of energy from the sun into electricity. Dye-sensitizer in DSCs plays a crucial role as the chlorophyll in plants; to harvest solar light and transfer the energy via electron transfer to a suitable material (TiO2 in this case) to produce electricity. The topic of interest for this thesis is to further enhance the photovoltaic performance and the robustness of DSCs by tuning the optical properties of the dye-sensitizer (Ruthenium complex, in this case) using several strategies including an extension of the π-conjugation system, an introduction of antenna molecules and a modification of the Ru-complex structure. This work focuses on the DSC device fabrication and photovoltaic characterization in order to investigate more insight into structure-property-device performance relationship. New benchmarks for high performance DSCs with ruthenium complex sensitizers with π-extension in their ancillary ligands were presented. The overall conversion efficiency of 9.6% and 8.5% have been achieved with Ru-based sensitizer containing ethylenedioxythiophene, using low-volatile electrolyte and solvent-free electrolyte, respectively. The Rusensitizer functionalized with hexylthio-bithiophene unit exhibited a conversion efficiency of 9.4% with low-volatile electrolyte. All these devices showed good stability under prolonged light soaking at 60 °C. Extending π-conjugation of the anchoring ligand with thiophene units in monoleptic Ru-sensitizer also yields an impressive conversion efficiency of 6.1% using 3-µm-thin mesoporous TiO2 film in corporate with low-volatile electrolyte. DSC devices based on ruthenium sensitizers functionalized with thienothiophene- and EDOT-conjugated bridge, together with carbazole moiety on the ancillary ligands were found efficient with conversion efficiencies of 9.4% and 9.6%, respectively, in presence of a volatile electrolyte. The carbazole-functionalized ruthenium-based DCSs also performed excellently in the stability test using a low-volatile electrolyte. Furthermore, the Ru-complexes synthesized by click-chemistry in association with triazole-derivative moieties were successfully used as DCS sensitizers. DSC devices sensitized with these dyes provided the overall conversion efficiency close to 10% with volatile electrolyte. Further studies with solvent-free electrolyte showed notable device stability under extending full sunlight intensity at 60 °C. The results presented here provide a fertile base for further investigation, which will focus on improving the spectral response of ruthenium dye-sensitizer to full sunlight by searching for new strategies to modify the sensitizer with more efficient functional groups. The target is to reach higher conversion efficiency of DSC devices while retaining their stability under standard reporting conditions.

Strategies to Optimizing Dye-Sensitized Solar Cells

Sophie Wenger

Dye-sensitized solar cells (DSCs) constitute a novel class of hybrid organic-inorganic solar cells. At the heart of the device is a mesoporous film of titanium dioxide (TiO2) nanoparticles, which are coated with a monolayer of dye sensitive to the visible region of the solar spectrum. The role of the dye is similar to the role of chlorophyll in plants; it harvests solar light and transfers the energy via electron transfer to a suitable material (here TiO2) to produce electricity — as opposed to chemical energy in plants. DSCs are fabricated of abundant and cheap materials using inexpensive processes (e.g. screen-printing) and are likely to be a significant contributor to the future commercial photovoltaic technology portfolio. The work conducted during this thesis aimed at optimizing the DSC using three different strategies: The use of versatile organic sensitizers for stable and efficient DSCs, the study of tandem device architectures in combination with other solar cells to harvest a larger fraction of the solar spectrum, and the development of a validated optoelectric model of the DSC. Organic donor-π-acceptor dyes are an interesting alternative to the standard metal-organic complexes used in DSCs. Efficient photovoltaic conversion and stable performance could be demonstrated with three classes of donor systems, namely diphenylamine, difluorenylaminophenyl, and π-extended tetrathiafulvalene. The highest conversion efficiencies were obtained with a difluorenylaminophenyl donor system (η = 8.3 % with a volatile electrolyte and η = 7.6 % with a solvent-free ionic liquid, which was a new record for organic dyes at the time of publication). Surprisingly, efficient regeneration of the oxidized dye by the I-/I3- redox mediator was found with the π-extended tetrathiafulvalene system, even though the thermodynamic driving force was as low as 150 mV. So far driving forces of 300-500 mV had been regarded as necessary for efficient regeneration of the dye cation. Also, important structure-property relationships pertaining to the recombination of electrons with the electrolyte and to the stability of the device could be identified (i.e. effect of linear vs. branched structure, linker length, and moieties used). The power conversion efficiency of solar cells can be extended beyond the limit for a single cell (∼ 30 %) by using multiple cells with different optical gaps in a tandem device. DSCs and chalcopyrite Cu(In,Ga)Se2 (CIGS) solar cells have complementary optical gaps and are thus suitable systems for integration in a tandem device. It was shown that a monolithic DSC/CIGS tandem device has the potential for increased efficiency over a mechanically stacked device due to increased light transmission to the bottom cell, and a monolithic DSC/CIGS device with an initial efficiency of η = 12.2 % was demonstrated. The degradation of the devices — induced by the corrosion of the CIGS cell in contact with the I-/I3- redox mediator — could be retarded with a protective thin conformal ZnO/TiO2 oxide layer coated on the CIGS cell by atomic layer deposition. Finally, an experimentally validated optical and electrical model of the DSC has been developed to assist the optimization process, which is predominantly conducted by empirical means in the DSC research community. The optical model allows to accurately calculate the internal quantum efficiency of devices, i.e. the ratio of extracted electrons to absorbed photons by the dye, a crucial and so far difficult to determine characteristic. Intrinsic parameters — like injection efficiency, electron diffusion length, or distribution of trap states in the TiO2 — can be extracted from experimental steady-state and time-dependent data with the electric model. The model allows to make a comprehensive and quantitative loss analysis of the different optical and electric loss channels in the DSC. The model has been implemented with a graphical user interface for straightforward usage. All three optimization strategies — organic dyes, tandem architecture, and device modeling — developed during this thesis make a valuable contribution to the development and commercialization of inexpensive and high efficiency DSCs. They enable a comprehensive view of the system and pave the way for a systematic analysis and reduction of losses, which has been the ultimate route to success for several established photovoltaic technologies.

EPFL2010

Electronic and structural modification of titanium dioxide/zinc oxide photoanode for dye-sensitized solar cells

Aravind Kumar Chandiran

Dye-sensitized solar cells (DSC) are a new class of molecular photovoltaics that mimics the natural photosynthesis, for the direct conversion of sunlight into electricity. A typical DSC is a sandwich of a dye sensitized nanoparticle TiO2 film and a catalyst coated counter electrode, with a redox electrolyte filling the space in between. The high internal surface area of the mesoporous TiO2 film ensures higher dye uptake for an efficient light harvesting. Following the light illumination, an electron from the highest occupied molecular orbitals (HOMO) of the dye is excited to its lowest unoccupied molecular orbitals (LUMO). Due to an effective electronic orbital coupling between dye and TiO2, the excited electrons are injected into TiO2. The oxidized dye is regenerated by the electron donor present in the electrolyte. The circuit is completed by the transfer of photogenerated electrons in TiO2 via an external load, to counter electrode regenerating the oxidized electrolyte species. In a conventional DSC, the transport rate of the photogenerated electrons in TiO2 competes with the parasitic electron recombination, as these two processes occur at a similar ms time domain. This thesis work is aimed at the fundamental understanding of these electron transfer processes at sensitized photoanodes that were subject to different bulk and surface electronic structural modifications. The implications were shown to be of profound interest for the fabrication of high efficiency and low cost dye-sensitized solar cells. In the first part of the thesis, the bulk electronic structure of the anatase TiO2 nanoparticles is modified by the incorporation of different concentrations of aliovalent cations (Nb5+, Ga3+, Y3+) in the crystal lattice. The cationic doping/substitution in the nanoparticles is achieved by hydrothermal process. The modified nanoparticles retained the parent anatase crystal structure of TiO2. The transparent dye-sensitized solar cells made with these modified films, using our standard heteroleptic metal complex sensitizers and iodide/triiodide redox electrolyte, displayed enhanced photovoltaic performances, at certain added concentrations. Charge extraction measurements in DSC, reveals a formation of deeper trap states below the conduction band of titanium dioxide when Nb ion is incorporated into the TiO2 lattice. On the other hand, Ga3+ and Y3+ did not significantly affect the trap states distribution compared to the reference device. The modification in the trap states and the oxygen stoichiometry due to these cation inclusions significantly influenced the electron recombination and transport rates. These properties are investigated in detail using transient decay techniques. Following the study on the electronic structure of the bulk TiO2, the influence of the surface modification of the TiO2 nanoparticles on the electron transfer dynamics is investigated. The kinetics of electron recombination from TiO2 to the single electron redox shuttles is fast, leading to a significant loss in the open-circuit potential (VOC) of devices. The emergence of cobalt complexes as a potential electrolyte motivated us to investigate the role of surface passivation of the titanium dioxide nanoparticles. Insulating sub-nanometer oxide tunneling layers deposited by atomic layer deposition (ALD) are known to block the electron recombination, thereby, leading to an increase in the VOC and the collection efficiency of the solar cell. A general perception in the DSC community is that any insulating oxide layer can block the recombination. However, in this work, it is unraveled that, just the insulating property of oxides is not sufficient. In addition, the properties such as the conduction band position and the oxidation state of the insulating oxide, the electronic structural modification induced to the underlying TiO2 mesoporous film, modification of surface charges (isoelectric point) and charge of the electrolyte species have to be considered. A complete photovoltaic study has been done by depositing different cycles (by ALD) of four insulating oxides (Ga2O3, ZrO2, Nb2O5 and Ta2O5) and their recombination characteristics, surface electronic properties, transport rate and injection dynamics are investigated with a standard organic dye and Co2+/Co3+ redox mediator. A comparison is made with the conventional iodide/triiodide electrolyte. The second part of the thesis is focused at the development of new type of photoanodes for effective electron transport to increase the charge collection efficiency. We present a way of utilizing thin and conformal overlayer of titanium dioxide or zinc oxide on an insulating mesoporous template as a photoanode for dye-sensitized solar cells. Different thicknesses of TiO2 ranging from 1 to 15 nm are deposited on the surface of the template by ALD. This systematic study helps unraveling the minimum critical thickness of the TiO2 overlayer required to transport the photogenerated electrons efficiently. A merely 6 nm thick TiO2 film on a 3 μm mesoporous insulating substrate is shown to transport 8 mA/cm2 of photocurrent density along with ≈900 mV of open-circuit potential, when using our standard donor-π-acceptor sensitizer and Co(bipyridine)3 redox mediator. The three dimensional conformal shell on the insulating core also exhibits one order of magnitude higher transport rate compared to the conventional TiO2 nanoparticles. Similar nanostructures were made by depositing ZnO on an insulating template and the electron transfer properties are compared with titania. We also developed ALD recipe for the deposition of crystalline TiO2 or ZnO at low temperatures (< 200 °C) on arbitrary mesoporous insulating templates, considering their prospects of making photoanodes on flexible plastic substrates. The experience gained about the deposition of conformal and pin-hole free TiO2 layer in the previous section is extended for the high efficiency solid-state mesoscopic solar cells based on CH3NH3PbI3 perovskite absorbers. We show that a 2 nm as-deposited ALD overlayer on a mesoporous TiO2 film, printed on a bare conducting glass, is sufficient to arrest the recombination process that lead to a power conversion efficiency of 11.5 %. A detailed investigation on the electron back reaction is presented using electrochemical impedance spectroscopy.

EPFL2013

Cyclometalated Ruthenium Complexes for Dye-Sensitized Solar Cells

Sadig Aghazada

The transition towards carbon-free sources of energy is vital for humankind. This process is feasible through the extensive use of photovoltaic technologies, which may convert solar energy into elec-trical and chemical energy. In the last half-century, many competitive technologies were developed. One promising type of solar cell is Dye-Sensitized Solar Cell (DSC), which differ from other solar cells in many ways. DSCs can be produced in different colors and used as constructing components of urban buildings. Additionally, DSCs have inner beauty related to their working principle. The pro-cesses of light absorption and charge transport are spatially separated in many ways reminiscent of water oxidation in Photosystem II in nature. Historically, DSCs were developed with an electrolyte based on an iodine-iodide redox shuttle. However, the overall performance of these solar cells is restricted due to the high loss of potential imparted by multistep dye regeneration by iodide. Shift to the outer-sphere one-electron redox couple, such as Co3+/2+ imine complexes, boosted the field, and new record efficiencies were achieved. Still, new redox mediators failed to provide reasonable power conversion efficiencies using the classic ruthenium sensitizers with isothiocyanate ligands. Poor performance is related to high charge recombination rates. To tackle this problem, I present my research on new cyclometalated ruthenium (II) complexes as sensitizers for DSCs that employ cobalt-based electrolytes. I introduce tris-heteroleptic cyclometalated Ru (II) complexes, which have various organic substituents. Optical and electrochemi-cal analyses are conducted before applying the new sensitizers in state-of-the-art solar cells. In this work, a record-high power conversion efficiency of 9.4 % was obtained with a ruthenium sensitizer and a cobalt-based redox shuttle. Transient absorbance spectroscopy, electrochemical impedance spec-troscopy, and transient photovoltage and photocurrent decay measurements were conducted to reveal the main factors that limit the performance of the solar cells. The role of dye-loading, which is generally ignored in the field, was shown to be crucial. Additionally, it was shown that organic substituents that are usually attached to improve the photophysical properties of sensitizers cause irreversible sensi-tizer electrochemical oxidation. Further, in this thesis, I discuss the role of sulfur atoms in the sensitizer structure on the performance of the solar cell. Then I introduce new bidentate ligands, which coordinate to the metal with both cyclometalation and N-heterocyclic carbenes. I investigate the potential of new complexes with presented ligands as sensitizers. In the last section, I discuss the extended anchoring ligands and their complexes with ruthenium, which have six-membered chelating rings. In this section I also introduce new binding mode for the pyridine-type of ligand. Finally, I briefly summarize the obtained results.

EPFL2018